Osteoarthritis (OA), especially at the knee, is a leading cause of chronic disability in the elderly. Few strategies to prevent knee OA disease progression or OA-related disability exist, in large part due to limited knowledge of factors responsible for these outcomes. In recent years, there has been a shift in epidemiological studies towards heavier focus on progression than on the initial development of knee OA. Also, recognition of the role in progression of local factors is growing, especially those factors that modify joint loading. Recent findings support that hip muscle forces may play an important role in knee joint protection in persons with osteoarthritic knees. In this application, we propose to examine, in persons with knee OA, the effect of hip muscle strength, particularly hip abductor and external rotator, on: cartilage loss and worsening of other joint tissues; physical function decline; and disability progression. The proposed aims build upon our previous results. A long-term goal of the proposed project is to inform intervention development that capitalizes upon the hip muscles to benefit persons with knee OA. We propose two evaluations (at baseline and 2 years later) of a cohort with knee OA, in which we will measure strength of hip muscles as well as collect covariate, radiographic, magnetic resonance imaging (MRI), functional status, and disability data, using state-of-the-art approaches to assess each outcome. The equipment, pulse sequences, and quality of the MRI protocol support quantitative assessment of articular cartilage. The results of this study will inform development of physical and rehabilitative therapy for knee OA that might take advantage of the hip musculature. This is a key, modifiable aspect of the knee joint environment that has received little attention in knee OA, despite a potentially large role of hip muscles in both knee-level and person-level function. The proposed study will include structural outcomes by MRI and by x-ray and person-relevant outcomes by self-report and task performance. Given the role played by the hip, it is plausible that hip muscle status will be linked not only to knee OA progression but also to associated function limitation and disability. Osteoarthritis (OA) at the knee is a major cause of disability. Few strategies to prevent OA disease progression or OA-related disability exist. We propose to examine in persons with knee OA the effect of hip muscle strength, particularly hip abductor and external rotator, on cartilage loss and worsening of other joint tissues, physical function decline, and disability progression. The results of the proposed study will inform therapy of knee OA that takes advantage of the hip musculature.